The energy consumptions worldwide have increased simultaneously with the growth of the population and of the economy. Nowadays, finding an alternative way to satisfy the energy demand is one of the great challenges for the future of humanity, especially due to the limitation of fossil fuels and their effect on global warming. Hydrogen, as an alternative fuel for the future, is very attractive. Compared to traditional methods, such as the steam reforming of natural gas or coal gasification, photocatalytic water splitting (PWS) is considered to be the most sustainable alternative for producing hydrogen as a future fuel. PWS usually relies on semiconductor material that can transform the absorbed solar photon into photogenerated electrons and holes, creating a photopotential which can drive the electrochemical production of molecular hydrogen from the reduction of water. Despite its promising application, semiconductor-based PWS usually suffers from low carrier mobility and short diffusion length. Furthermore, the recombination of photogenerated electrons and holes might occur, especially if there are no suitable reaction sites available on the surface of the semiconductor. In order to facilitate the catalytic reactions on the surface of the semiconductor, the presence of a cocatalyst is necessary in order to obtain more efficient PWS processes. To this day, noble metals such as Pt, Pd, RuO2 and IrO2 are still the benchmark cocatalysts for PWS. Nevertheless, due to their high cost and limited supply, it is mandatory to develop a suitable strategy and to identify more efficient materials. Therefore, within the framework of this dissertation, novel cocatalysts and strategies that can improve the efficiency of the photocatalytic water splitting processes have been developed. Firstly, we developed a cocatalyst combining noble metals and semiconductors by means of partial galvanic replacement of the Cu2O nanocrystal with Au. The deposition of this cocatalyst on TiO2 was studied for the photocatalytic H2 production in order to explore the synergistic effect of the plasmonic resonance from the Au nanoparticles and pn-junction between Cu2O and TiO2. Additionally, the plasmonic effect of the Au films was also studied and utilized in order to improve the PWS. Secondly, the nanoscaling of cocatalysts was studied in order to improve the efficiency thereof and to reduce the cost of the cocatalyst materials. Moreover, it is sought to explore the quantum size effect on the properties of the cocatalyst and their effect on the photocatalytic reaction. Atomically precise Au and Ni nanoclusters were employed in these studies. Au nanoclusters were studied in relation to the cocatalysts in the photocatalytic water splitting, and Ni nanoclusters were studied in relation to the cocatalysts in the electrocatalytic water oxidation. The results of these studies will provide new insights in relation to the strategy used in order to develop efficient cocatalysts for the purpose of photocatalytic water splitting.

No description available.

Nanoclusters de metais de transição (TM) com dimensões de &#126;1,0 nm têm atraído grande interesse em diversas aplicações tecnológicas, como microeletrônica, óptica, dispositivos magnéticos e principalmente como nanocatalisadores. Nesse contexto, a morfologia desses sistemas é um fator fundamental para otimzação de performace nessas aplicações, tendo em vista a relação direta entre as propriedades físico-químicas e a estrutura atômica do nanocluster determinada pelas condições do ambiente (temperatura e moléculas adsorvidas). No entanto, diante das limitações relativas à precisão das técnicas de caracterização experimental disponíveis atualmente, sobretudo em dimensões nanométricas, o uso de cálculos computacionais através de métodos de primeiros princípios (ab initio) e baseados na teoria do funcional da densidade se torna indispensável. Neste trabalho, foram investigados os efeitos de ligantes e temperaturas nas propriedades estruturais, eletrônicas, estabilidade e meta-estabilidade de nanoclusters de TM. (i) Os efeitos de ligantes de (PH3)n e (SH2)n (saturação gradativa em n = 1, 6, 12 e 18) adsorvidos em nanoclusters unários de Pt55 e Au55 com estruturas icosaédricas (ICO), cuboctaédricas (CUB) e desordenadas de core reduzido (DRC) foram estudados a 0K. Em fase gasosa, as estruturas DRC com 7 e 9 átomos na região do core são 5,34 eV (Pt55) e 2,20 eV (Au55) mais estáveis que modelo ICO com simetria Ih com 13 átomos no core. Os resultados mostraram que existe uma forte compressão do core catiônico pela superfície aniônica induzida por interações de Coulomb (core+-superfície-), levando ao colapso e redução de estresse das estruturas simétricas a partir da redução do número de átomos na região do core. No entanto, a estabilidade da estrutura ICO aumenta com o aumento do número de moléculas adsorvidas, de modo que DRC e ICO se tornam energeticamente degeneradas em < 0,5 eV. Além disso, a adição de ligantes na superfície aniônica reduz a transferência de cargas entre as regiões de core+-superfície-, contribuindo para a redução das interações de Coulomb e, consequentemente, aliviando o estresse interno da estrutura ICO. Resultados similares foram obtidos utilizando ligantes de trifenilfosfina (PPh3), nos quais as longas cadeias carbônicas adicionam interações laterais entre os ligantes. (ii) Nanoclusters de binários de PtnCo(55 - n) a 0K mostraram alta estabilidade em todo intervalo de composições, indicando uma correlação direta entre estabilidade e distribuição homogênea de átomos de Pt e Co formando a nanoliga com estrutura ICO. No entanto, sob uma atmosfera saturada de ligantes de CO adsorvidos, a estabilidade da nanoliga diminui (aumenta) para composições com grandes quantidades de Pt (Co). As análises mostraram que sob os efeitos da adsorção as composições permanecem com configuração ICO, exceto para Pt42Co13 (estrutura core@shell do tipo Co@Pt em fase gasosa), onde as moléculas de CO deslocam os átomos de Co para a superfície, e, então, induzindo um processo de amorfização na formação de uma estrutura formada com átomos de Pt ocupando o core. Para investigar os efeitos de temperatura na estabilidade e meta-estabilidade de nanoclusters e clusters de Au utilizando dinâmica molecular de Born-Oppenheimer, foram utilizados nanoclusters de tamanho médio de Au25, Au38 e Au40 em fase gasosa e clusters de Au13 sob diferentes atmosferas saturadas de CO. (iii) Observou-se que em temperaturas de 300, 400, 500 e 600 K os nanoclusters exibem estruturas dinâmicas para as regiões de core-superfície, com core tetraédrico (4 átomos) ou T-bipiramidal (5 átomos) catiônicos fracamente ligados à superfície aniônica flexível. Uma abordagem estatística através de um algoritmo de redução de dimensionalidades para representação no espaço euclideano bidimensional, chamado de sketch-map, foi proposta como uma nova linguagem para analisar a superfície de energia livre (FES) dos sistemas calculada na aproximação de multistate Bennet acceptance-ratio. A FES foi utilizada como uma análise qualitativa das configurações estáveis e meta-estáveis sob os efeitos de temperaturas, indicando as regiões preferenciais do espaço configuracional de cada nanocluster investigado. (iv) Incluindo os efeitos de ligantes e temperaturas em clusters de Au13, foi observado que em altas temperaturas os clusters de Au13 em fase gasosa tendem a configurações mais abertas com menor comprimento de ligação com relação às estruturas tridimensionais. Por outro lado, a saturação do ambiente com ligantes de CO restringem a mobilidade dos átomos nos clusters de Au13, favorecendo a amostragem de estruturas tridimensionais mesmo em temperaturas relativamente altas. A análise de população de cargas mostrou uma alta concentração de elétrons nos átomos de O, de modo que o forte catiônico dos clusters de Au13 leva ao aumento da coordenação atômica, contribuindo para a estabilização das estruturas mais tridimensionais. / Transition metal nanoclusters (TM) with dimensions of &#126;1,0 nm have attracted great interest in various technological applications such as microelectronics, optics, magnetic devices and mainly as nanocatalysts. In this way, the morphology of these systems is a fundamental factor for optimization of performance in these applications, considering the direct relationship between the physicochemical properties and the atomic structure of the nanocluster determined by the ambient conditions (temperature and adsorbed molecules). In this sense, in view of the limitations on the precision of the experimental characterization techniques currently available, especially in nanometric dimensions, the use of computational calculations using first principles methods (ab initio) and based on the density functional theory is indispensable. Thus, in this work, the effects of binders and temperatures on the structural, electronic properties, stability and metastability of TM nanoclusters were investigated. (i) In this way, the (PH3)n and (SH2)n ligands effects with gradual saturation at n = 1, 6, 12 and 18 adsorbed onto nanoclusters Pt55 and Au55 with icosahedral (ICO), cuboctahedron (CUB), and distorted reduced core (DRC) structures were studied at 0K. In the gas phase, the DRC structures with 7 and 9 atoms in the core region are 5,34 eV (Pt55) and 2,20 eV (Au55) more stable than ICO model with symmetry Ih with 13 atoms in core. The results showed that there is a strong compression of the cationic core by the anionic surface induced by interactions of Coulomb (core+-surface-), leading to collapse and stress reduction of the symmetrical structures from the reduction of the number of atoms in the core region. However, the stability of the ICO structure increases with increasing Number of molecules adsorbed, so that DRC and ICO become approximately degenerated in energy in < 0,5 eV. In addition, ligands on the anionic surface reduces the transfer of charges between core+- surface-, contributing to the reduction of interactions of Coulomb and, consequently, alleviating the internal stress of the ICO. Similar result were obtained using triphenylphosphine (PPh3) as large ligands, in which the long carbon chains add side interactions between the ligands. (ii) PtnCo(55 - n) binares nanoclusters at 0K showed a high stability across the range of compositions, indicating a direct correlation between stability and homogeneous distribution of Pt and Co atoms forming the nanoalloys with ICO structure. However, under a saturated atmosphere of adsorbed CO ligands, the stability of the nanoaaloys decreases (increases) to compositions with large amounts of Pt (Co). Analyzes have shown that under the effects of adsorption the compositions remain with ICO configuration, except for Pt42Co13 (Co@Pt as core@shell in gas-phase), where Co molecules displace the Co atoms to the surface, and then inducing an amorphization process for a structure formed with Pt atoms occupying the core. To investigate the temperature effects on the stability and metastability of Au nanoclusters and clusters by using Born-Oppenheimer molecular dynamics, medium size nanoclusters of Au25, Au38, and Au40 were used in gas-phase and clusters of Au13 under different saturated atmospheres of CO. (iii) It has been observed that at temperatures of 300, 400, 500, and 600 K, the nanoclusters exhibit dynamic structures for the core-surface regions with cationic tetrahedral (4 atoms) or T-bipyramidal (5 atoms) cores weakly bound to the flexible anionic surface. A statistical approach using a dimensionality reduction algorithm for two-dimensional Euclidean space representation, called sketch-map, was proposed as a new language to analyze the free energy surface (FES) of the systems calculated on the approximation of multistate Bennet acceptance-ratio. The FES was used as a qualitative analysis of the stable and metastable configurations under the effect of temperature, indicating the preferred regions of the configurational space of each nanocluster investigated. (iv) By including the effects of ligands and temperatures on Au13 clusters, it was observed that at high temperatures the Au13 clusters in gas-phase tend to have more open configurations with less bond length relative to three-dimensional structures. On the other hand, the saturation of the environment with CO ligands restricts the mobility of the atoms in Au13 clusters, favoring the sampling of three-dimensional structures even at temperatures relatively high. The charge population analysis showed a high concentration of electrons in the O atoms, so that the strong cationic character of the Au13 clusters leads to increased atomic coordination, contributing to the stabilization of the more three-dimensional.

Mes travaux de recherche concernent la synthèse et la caractérisation des nanoclusters stabilisés pardes ligands thiolés, et plus particulièrement des nanoclusters d'or et d'argent. L'étude de ces clusterspar des méthodes de caractérisation optique en solution, séparation sur le gel d'électrophorèse et laspectrométrie de masse à haute résolution a permis de sonder quelques propriétés structurelles etélectroniques. L'originalité de ce travail de thèse vient, du couplage d'un spectromètre de masse avecun laser accordable en longueur d'onde UV/Visible pour étudier les propriétés optiques de clusters enphase gazeuse. En effet, ce travail a permis d'obtenir le premier spectre sur des mesures optiques deces espèces.Une partie de ce travail est consacrée à la synthèse et la caractérisation des nanoclusters d'or stabiliséspar les glutathions allant de Au4(SG) 4 à Au25(SG) 18. Les expériences de fragmentation induite parcollision sont réalisées sur les clusters afin de sonder leurs structures. Nous avons développé unenouvelle méthode basée sur l'analyse des structures isotopiques expérimentales pour déterminer lenombre d'électrons accommodés le coeur métallique.Ces études expérimentales ont été étendues à d'autres systèmes tels que les clusters d'argent stabiliséspar les glutathions. Deux méthodes de synthèse des clusters Agx(SG)y ont été développées aulaboratoire. Dans le cadre de ce travail, pour la première fois, nous avons pu déterminer la stichométriede ces clusters Ag31(SG) 19 et Ag15(SG) 11.

Mes travaux de recherche concernent la synthèse et la caractérisation des nanoclusters stabilisés pardes ligands thiolés, et plus particulièrement des nanoclusters d'or et d'argent. L'étude de ces clusterspar des méthodes de caractérisation optique en solution, séparation sur le gel d'électrophorèse et laspectrométrie de masse à haute résolution a permis de sonder quelques propriétés structurelles etélectroniques. L'originalité de ce travail de thèse vient, du couplage d'un spectromètre de masse avecun laser accordable en longueur d'onde UV/Visible pour étudier les propriétés optiques de clusters enphase gazeuse. En effet, ce travail a permis d'obtenir le premier spectre sur des mesures optiques deces espèces.Une partie de ce travail est consacrée à la synthèse et la caractérisation des nanoclusters d'or stabiliséspar les glutathions allant de Au4(SG) 4 à Au25(SG) 18. Les expériences de fragmentation induite parcollision sont réalisées sur les clusters afin de sonder leurs structures. Nous avons développé unenouvelle méthode basée sur l'analyse des structures isotopiques expérimentales pour déterminer lenombre d'électrons accommodés le coeur métallique.Ces études expérimentales ont été étendues à d'autres systèmes tels que les clusters d'argent stabiliséspar les glutathions. Deux méthodes de synthèse des clusters Agx(SG)y ont été développées aulaboratoire. Dans le cadre de ce travail, pour la première fois, nous avons pu déterminer la stichométriede ces clusters Ag31(SG) 19 et Ag15(SG) 11.

Nanoclusters de metais de transição (TM) com dimensões de &#126;1,0 nm têm atraído grande interesse em diversas aplicações tecnológicas, como microeletrônica, óptica, dispositivos magnéticos e principalmente como nanocatalisadores. Nesse contexto, a morfologia desses sistemas é um fator fundamental para otimzação de performace nessas aplicações, tendo em vista a relação direta entre as propriedades físico-químicas e a estrutura atômica do nanocluster determinada pelas condições do ambiente (temperatura e moléculas adsorvidas). No entanto, diante das limitações relativas à precisão das técnicas de caracterização experimental disponíveis atualmente, sobretudo em dimensões nanométricas, o uso de cálculos computacionais através de métodos de primeiros princípios (ab initio) e baseados na teoria do funcional da densidade se torna indispensável. Neste trabalho, foram investigados os efeitos de ligantes e temperaturas nas propriedades estruturais, eletrônicas, estabilidade e meta-estabilidade de nanoclusters de TM. (i) Os efeitos de ligantes de (PH3)n e (SH2)n (saturação gradativa em n = 1, 6, 12 e 18) adsorvidos em nanoclusters unários de Pt55 e Au55 com estruturas icosaédricas (ICO), cuboctaédricas (CUB) e desordenadas de core reduzido (DRC) foram estudados a 0K. Em fase gasosa, as estruturas DRC com 7 e 9 átomos na região do core são 5,34 eV (Pt55) e 2,20 eV (Au55) mais estáveis que modelo ICO com simetria Ih com 13 átomos no core. Os resultados mostraram que existe uma forte compressão do core catiônico pela superfície aniônica induzida por interações de Coulomb (core+-superfície-), levando ao colapso e redução de estresse das estruturas simétricas a partir da redução do número de átomos na região do core. No entanto, a estabilidade da estrutura ICO aumenta com o aumento do número de moléculas adsorvidas, de modo que DRC e ICO se tornam energeticamente degeneradas em < 0,5 eV. Além disso, a adição de ligantes na superfície aniônica reduz a transferência de cargas entre as regiões de core+-superfície-, contribuindo para a redução das interações de Coulomb e, consequentemente, aliviando o estresse interno da estrutura ICO. Resultados similares foram obtidos utilizando ligantes de trifenilfosfina (PPh3), nos quais as longas cadeias carbônicas adicionam interações laterais entre os ligantes. (ii) Nanoclusters de binários de PtnCo(55 - n) a 0K mostraram alta estabilidade em todo intervalo de composições, indicando uma correlação direta entre estabilidade e distribuição homogênea de átomos de Pt e Co formando a nanoliga com estrutura ICO. No entanto, sob uma atmosfera saturada de ligantes de CO adsorvidos, a estabilidade da nanoliga diminui (aumenta) para composições com grandes quantidades de Pt (Co). As análises mostraram que sob os efeitos da adsorção as composições permanecem com configuração ICO, exceto para Pt42Co13 (estrutura core@shell do tipo Co@Pt em fase gasosa), onde as moléculas de CO deslocam os átomos de Co para a superfície, e, então, induzindo um processo de amorfização na formação de uma estrutura formada com átomos de Pt ocupando o core. Para investigar os efeitos de temperatura na estabilidade e meta-estabilidade de nanoclusters e clusters de Au utilizando dinâmica molecular de Born-Oppenheimer, foram utilizados nanoclusters de tamanho médio de Au25, Au38 e Au40 em fase gasosa e clusters de Au13 sob diferentes atmosferas saturadas de CO. (iii) Observou-se que em temperaturas de 300, 400, 500 e 600 K os nanoclusters exibem estruturas dinâmicas para as regiões de core-superfície, com core tetraédrico (4 átomos) ou T-bipiramidal (5 átomos) catiônicos fracamente ligados à superfície aniônica flexível. Uma abordagem estatística através de um algoritmo de redução de dimensionalidades para representação no espaço euclideano bidimensional, chamado de sketch-map, foi proposta como uma nova linguagem para analisar a superfície de energia livre (FES) dos sistemas calculada na aproximação de multistate Bennet acceptance-ratio. A FES foi utilizada como uma análise qualitativa das configurações estáveis e meta-estáveis sob os efeitos de temperaturas, indicando as regiões preferenciais do espaço configuracional de cada nanocluster investigado. (iv) Incluindo os efeitos de ligantes e temperaturas em clusters de Au13, foi observado que em altas temperaturas os clusters de Au13 em fase gasosa tendem a configurações mais abertas com menor comprimento de ligação com relação às estruturas tridimensionais. Por outro lado, a saturação do ambiente com ligantes de CO restringem a mobilidade dos átomos nos clusters de Au13, favorecendo a amostragem de estruturas tridimensionais mesmo em temperaturas relativamente altas. A análise de população de cargas mostrou uma alta concentração de elétrons nos átomos de O, de modo que o forte catiônico dos clusters de Au13 leva ao aumento da coordenação atômica, contribuindo para a estabilização das estruturas mais tridimensionais. / Transition metal nanoclusters (TM) with dimensions of &#126;1,0 nm have attracted great interest in various technological applications such as microelectronics, optics, magnetic devices and mainly as nanocatalysts. In this way, the morphology of these systems is a fundamental factor for optimization of performance in these applications, considering the direct relationship between the physicochemical properties and the atomic structure of the nanocluster determined by the ambient conditions (temperature and adsorbed molecules). In this sense, in view of the limitations on the precision of the experimental characterization techniques currently available, especially in nanometric dimensions, the use of computational calculations using first principles methods (ab initio) and based on the density functional theory is indispensable. Thus, in this work, the effects of binders and temperatures on the structural, electronic properties, stability and metastability of TM nanoclusters were investigated. (i) In this way, the (PH3)n and (SH2)n ligands effects with gradual saturation at n = 1, 6, 12 and 18 adsorbed onto nanoclusters Pt55 and Au55 with icosahedral (ICO), cuboctahedron (CUB), and distorted reduced core (DRC) structures were studied at 0K. In the gas phase, the DRC structures with 7 and 9 atoms in the core region are 5,34 eV (Pt55) and 2,20 eV (Au55) more stable than ICO model with symmetry Ih with 13 atoms in core. The results showed that there is a strong compression of the cationic core by the anionic surface induced by interactions of Coulomb (core+-surface-), leading to collapse and stress reduction of the symmetrical structures from the reduction of the number of atoms in the core region. However, the stability of the ICO structure increases with increasing Number of molecules adsorbed, so that DRC and ICO become approximately degenerated in energy in < 0,5 eV. In addition, ligands on the anionic surface reduces the transfer of charges between core+- surface-, contributing to the reduction of interactions of Coulomb and, consequently, alleviating the internal stress of the ICO. Similar result were obtained using triphenylphosphine (PPh3) as large ligands, in which the long carbon chains add side interactions between the ligands. (ii) PtnCo(55 - n) binares nanoclusters at 0K showed a high stability across the range of compositions, indicating a direct correlation between stability and homogeneous distribution of Pt and Co atoms forming the nanoalloys with ICO structure. However, under a saturated atmosphere of adsorbed CO ligands, the stability of the nanoaaloys decreases (increases) to compositions with large amounts of Pt (Co). Analyzes have shown that under the effects of adsorption the compositions remain with ICO configuration, except for Pt42Co13 (Co@Pt as core@shell in gas-phase), where Co molecules displace the Co atoms to the surface, and then inducing an amorphization process for a structure formed with Pt atoms occupying the core. To investigate the temperature effects on the stability and metastability of Au nanoclusters and clusters by using Born-Oppenheimer molecular dynamics, medium size nanoclusters of Au25, Au38, and Au40 were used in gas-phase and clusters of Au13 under different saturated atmospheres of CO. (iii) It has been observed that at temperatures of 300, 400, 500, and 600 K, the nanoclusters exhibit dynamic structures for the core-surface regions with cationic tetrahedral (4 atoms) or T-bipyramidal (5 atoms) cores weakly bound to the flexible anionic surface. A statistical approach using a dimensionality reduction algorithm for two-dimensional Euclidean space representation, called sketch-map, was proposed as a new language to analyze the free energy surface (FES) of the systems calculated on the approximation of multistate Bennet acceptance-ratio. The FES was used as a qualitative analysis of the stable and metastable configurations under the effect of temperature, indicating the preferred regions of the configurational space of each nanocluster investigated. (iv) By including the effects of ligands and temperatures on Au13 clusters, it was observed that at high temperatures the Au13 clusters in gas-phase tend to have more open configurations with less bond length relative to three-dimensional structures. On the other hand, the saturation of the environment with CO ligands restricts the mobility of the atoms in Au13 clusters, favoring the sampling of three-dimensional structures even at temperatures relatively high. The charge population analysis showed a high concentration of electrons in the O atoms, so that the strong cationic character of the Au13 clusters leads to increased atomic coordination, contributing to the stabilization of the more three-dimensional.

Keggin-based aluminum nanoclusters have been noted to be efficient sorbents for the adsorption of arsenic, copper, lead, and zinc from water. Obtaining a molecular-level understanding of the adsorption processes associated with these molecules is of fundamental importance and could pave the way for rational design strategies for water treatment. Furthermore, due to their size and the availability of experimental crystal structures, Al nanoclusters are computationally tractable at the atomistic modeling level. The adsorption of contaminants onto metal-oxide surfaces with nanoscale Keggin-type structural topologies has been established, but identification of the reactive sites and the exact binding mechanism are lacking. In more common surface studies the two main factors that affect reactivity have been found to be charge and functional group identity. Since Al nanoclusters each have a distinct shape we introduce the effects of shape as a third factor. In all the work presented in this dissertation, it is extremely apparent that the shape of the aluminum particle plays the most important role in nanoparticle reactivity studies. We first focus on the reactivity of three aluminum polycations: [Al13O4(OH)24(H2O)12]7+ (Al13), [Al30O8(OH)56(H2O)26]18+ (Al30), and [Al32O8(OH)60(H2O)30]20+ (Al32). Using outer-sphere adsorption of sulfate and chloride as probe adsorbents, density functional theory (DFT) calculations determined that the reactivity can be represented as a function of particle topology, and not functional group identity or charge. Further exploring the shape-reactivity relationship of Al30 we reveal that cations and anions have opposing trends and ion reactivity can be generalized. It is determined that all cations favor the adsorption sites on the caps of Al30 and all anions favor adsorption in the beltway (middle) region. This result is supported by the visualization of the electrostatic potential of Al30 and three-dimensional induced charge density maps. The middle of the cluster is more positive than the caps, and this promotes anion adsorption in the beltway and cation adsorption on the caps. Next we explore the reactivity of co-adsorption (outer-sphere anions and inner-sphere cations) onto Al30 through a collaborative approach. Al30 with two surface-bound Cu2+ cations (Cu2Al30-S) was experimentally crystallized in the presence of disulfonate anions; however, in the Cu2Al30-S structure the cations bind to the beltway region of the cluster. Using DFT we determined that the counter anions play a key (and governing) role in the crystallization of Cu2Al30-S. This result that outer-sphere adsorption dictates inner-sphere adsorption does not appear in surface calculations, it is unique to Keggin studies. Seeing that all anions favor adsorption to the beltway region and all cations favor adsorption to the cap region we set out to determine if any reactivity patterns can be reversed. In order to do this inner-sphere As(V) and P(V) adsorption is modeled onto Al30 through another collaborative approach. The experimental crystal structure of (TBP)2[Al2(μ4-O8)(Al28(μ2-OH)56(H2O)26)]14+ (where TBP = t-butylphosphonate (CH3)3CPO3) has been synthesized, and using DFT calculations we can alter the R-group of P(V) or the DFT As(V) analogue to see if the inner-sphere anion ever prefers to bind to the cap region instead of the beltway. We observe that no matter the intrinsic properties of the R-group the anion always prefers to bind to the beltway region, which once again shows that the shape-reactivity relationship plays a major role in Keggin based structure reactivity. Since As(V) is such a harmful ion we extend our As(V) adsorption studies to aluminum surfaces. As(V) has been experimentally shown to bind to aluminum surfaces in a bidentate binuclear configuration. By modeling a variety of configurations we can confirm and explain that the bidentate binuclear configuration is most stable due to the least amount of strain on the As(V) atom. Aluminum surfaces are common DFT models to study but are computationally expensive, due to this fact some people choose to model small Al octahedral cluster models instead. Comparing the reactivity of both systems we see a significant difference in energy magnitudes and ranges and can conclude that small Al octahedral cluster models cannot take place of aluminum surfaces. All in all, the work presented in this dissertation provides an important contribution in our understanding of Keggin based Al compounds. Keggin based compounds are very sparsely studied computationally and this work helps to fill a knowledge gap. Hopefully the insights obtained from this work can help guide future Keggin based studies.

Gold Nanoclusters are of particular interest due to their many possible applications across a wide range of scientific fields. More specifically, nano-sized gold particles have potential to be used in drug delivery systems, cancer therapy and catalysis. This dissertation focuses on improving our understanding of ligated gold nanoclusters by examining the role of a variety of phosphine based ligands, novel methods to produce monodisperse solutions, and investigating the kinetics of water soluble ligated gold nanoclusters. The addition of ligands to solutions of Au have shown to produce small (< 20 Au atoms) clusters. All nanocluster solutions were prepared in a similar manner. Typically, a gold salt, either Chloro(triphenylphosphine) gold(I) (Au(PPh3)Cl), or Potassium gold (III) chloride (KAuCl4), were dissolved in various solvents. Next, an equal concentration of ligand was added to the solution and stirred until completely dissolved. Finally, all were reduced with 5X the concentration of borane-tert-butylamine (BTBC) after which were sonicated for ~20 minutes. The timing and method of adding the ligands and reducing agent varied depending on the solution and solubility of the ligands. Primarily we used Electrospray Ionization Mass Spectrometry (ESI-MS) and Ultraviolet – Visible Spectroscopy (UV-VIS) for the characterization of samples, however, occasionally Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD) and X-Ray Photoemission Spectroscopy (XPS) were used. The most recent research took advantage of the size selective nature of an alpha hemolysin (a-HL) nanopore to investigate the kinetics of thiol-ligated Au clusters ~2 nm in size. The relationship between ligand rigidity and solvent polarity and the size and dispersity of Au cluster suspensions was investigated. We observed the formation of stable monodisperse clusters with the shortest ligand, (L3), independent of solvent. With a longer flexible ligand, (L6), we observed primarily Au8-10 cores depending on the ratio of L6/PPh3. All other ligands yielded polydisperse distributions. These dispersions contained clusters with a nuclearity between 8 and 11, for example [Au10(PPh3)9]3+ in LBn and [Au8(PPh3)7]2+ in LBp, were observed in the initial stages, but they were not stable and precipitated out or plated the glass vial. We also observed that the polarity of the solvent did not play a significant role in the formation of MPC’s, however a correlation between the size of the solvent and MPC formation was observed. The growth and evolution of two unique gold structures was also observed via UV-Vis and ESI-MS. Solutions were prepared which contained Potassium gold (III) chloride and PPh2(CH2)3PPh2, i.e., 1,3-bis(diphenylphosphino)propane, denoted by L3, reduced with Borane tert-butylamine complex (BTBC) in a 1:1 diethyl ether:methanol mix, which yielded a stable [Au11(L3)5]+3. Starting with this known Au11 solution, the addition of Mn2+ has shown to lead to the formation of a stable diphosphine ligated Au8 and a new Au14 species. Additionally, we show that the co-reduction of NiCl2 and Au in the absence of the ligand (L3) does give us a simple method for the production of a monodisperse [Au9(PPh3)8]2+ cluster solution and via electroless deposition does give us a potential low temperature pathway to the formation of a AuNi nanoalloy particle.

Charge transport dynamics through molecular scale materials is of common interest to both scientific and engineering disciplines. Putting molecules on nanoscale break junctions is the most straightforward setup to study charge transport dynamics through single molecules. Electromigration process can provide a simple and easy method of forming metallic oxide nanogap junctions. By using silver oxide thin films to form such nanogap junctions, silver nanoclusters (Ag2~Ag8) are also formed in-situ within the junctions. Formed silver nanoclusters strongly and stably electroluminesce under DC, AC, and customized pulse train excitation. By detecting extremely sensitive feedback, i.e. photons, two interesting behaviors of single molecule charge transport dynamics were revealed: 1) asymmetric charge transport and 2) discrete energy level tunneling. The discrete energy level tunneling of field emitted electrons yields negative differential resistance (NDR). Combined with photoconductivity and optical reduction of silver oxide to form silver nanoclusters, junction-asymmetry and NDR can be very useful in both electronic and optoelectronic applications such as on-demand electronics fabrication, single photon sources, and nanoscale photon detectors.

Water-soluble fluorescent metallic clusters at sizes comparable to the Fermi wavelength of an electron (~0.5 nm for gold and silver) were created and their photophysical properties were investigated at the bulk and single molecule levels. We employed biocompatible dendrimer and peptide to prepare a series of strong fluorescent gold and silver clusters with chemical or photo reduction methods. Facilitated by the well-defined dendrimer size, electrospray ionization mass spectrometry indicates that the fluorescent silver nanocluster size ranges from 2 to 8 Ag atoms. The correlation of emission energy with the number of atoms, N, in each gold nanocluster is quantitatively fit for the smallest nanoclusters with no adjustable parameters by the simple scaling relation of EFermi/N1/3, in which EFermi is the Fermi energy of bulk gold. The transition energy scaling inversely with cluster radius indicates that electronic structure can be well described with the spherical jellium model and further demonstrates that these nanomaterials are multi-electron artificial atoms. Fluorescence from these small metal clusters can be considered protoplasmonic, molecular transitions of the free conduction electrons before the onset of collective dipole oscillations occurring when a continuous density of states is reached. In addition, very strong single molecular Stokes and Antistokes Raman enhancement by fluorescent silver clusters was observed. Pushing to larger sizes, we also created ~ 2nm diameter glutathione encapsulated luminescent gold nanoparticles. Distinct from similarly sized but nonluminescent gold nanoparticles, these 2 nm gold nanoparticles show bright, long lifetime emission but no plasmon absorption. The emission might arise from charge transfer between gold atoms and the thiol ligand. Providing the missing link between atomic and nanoparticle behavior in noble metals, these highly fluorescent, water-soluble gold and silver nanoclusters offer complementary transition energy size scalings at smaller dimensions than do semiconductor quantum dots. The unique discrete excitation and emission and strong Stokes and antistokes Raman enhancement coupled with facile creation in aqueous solution open new opportunities for noble metal nanoclusters as biological labels, energy transfer pairs, and other light emitters in nanoscale electronics.